This invention relates generally to an ink jet recording apparatus which ejects ink through a plurality of nozzles supplied by an ink reservoir, and especially to a thermal ink jet recording apparatus which ejects ink through a plurality of nozzles without the need for separators between nozzles and an improved ink composition for use in the apparatus.
Thermal ink jet recording apparatus are well known in the art and provide high speed and high density ink jet printing having a relatively simple construction. Conventional ink jet recording apparatus and methods are described in the May, 1985 issue of the Journal of the U.S. Hewlett-Packard Company, hereinafter referred to as the Hewlett-Packard Journal, as well as in U.S. Pat. Nos. 4,359,079; 4,463,359; 4,528,577; 4,568,593 and 4,587,534.
The recording speed and density at which such conventional ink jet recording apparatus operate is limited. In order to protect the pressure of the heated ink underneath any one particular nozzle from affecting the pressure of ink under an adjacent nozzle, a barrier is placed between adjacent nozzles to prevent pressure interference. These barriers must be very thin in order to accommodate a plurality of nozzles on one recording head. Nevertheless, the pitch (i.e., spacing) between adjacent nozzles is still limited because of the need to place a barrier, no matter how thin, between each adjacent nozzle.
Additionally, thin film circuitry is covered by a protective layer of a hard insulated inorganic matter for protecting the heating elements and electrodes which are used to heat the ink from electrical, chemical, thermal and/or acoustic damage. This protective layer acts as a heat sink requiring more heat than would otherwise be required in order to reheat the ink to an appropriate temperature for ejection of the ink through the nozzles. This requires a longer period of time to heat the ink thereby reducing the speed at which the apparatus records. Further, small structural defects such as minute cracks in the protective layer can leave the thin film circuitry unprotected. Since it is difficult to produce protective layers without such small structural defects, the reliability of conventional thermal ink jet apparatus can be quite low.
It is also difficult to control the thickness of the protective layer during its manufacture. The thicker the protective layer, the less responsive the protective layer is to changes in the temperature of the heating element which it covers. Consequently, the heating element cools off much more quickly than the protective layer resulting in the ink adhering to the protective layer. As ink begins to build up heat conduction from the heating element to the ink is adversely affected and can eventually result in the inability to cause the ejection of ink through the nozzles.
The ink jet recording apparatus described in the Hewlett-Packard Journal includes a nozzle plate which covers a substrate on which the electrodes and heating elements are disposed. This nozzle plate, which is made by Ni electroforming, includes minute projecting portions provided on the interior surface thereof for ensuring that a gap of a predetermined height exists between the nozzle plate and substrate. The height of the gap is important to the operation of the ink jet recording apparatus since the amount of ink to be heated depends on the ink trapped within the gap. These minute projecting portions also must be of uniform height to ensure that the ink ejected through each nozzle inpinges the recording medium with the same desired inpact. In view of the foregoing it is essential to provide these minute projecting portions which makes manufacture of the nozzle plate difficult. The substrate and nozzle plate are adhesively bonded. The adhesive bonding material which deteriorates when contacted by ink and deposits can clog the nozzles of the nozzle plate adversely affecting the operation of the apparatus.
In addition to the above problems, the recording paper used for prior art ink jet recording apparatus varies significantly in the pulp, filler or other materials which are contained therein and in its manufacturing process (e.g., wire part, size press). Wood free paper such as described in the Hewlett-Packard Journal is widely used for ink jet recording apparatus. Other wood free paper applicable for use as a recording medium for thermal ink jet recording apparatus include, Japanese Industrial Standards for print A, drawing paper (such as document and Kent paper) and coated paper. Unfortunately, conventional ink has a tendency to significantly blot/spatter wood free paper and thus hinders achieving high quality printing.
Accordingly, it is desirable to provide an ink jet printer having a simplified construction which eliminates the need for barriers between adjacent nozzles. It is also desirable to provide an ink composition suitable for use in the ink jet printer which avoids the blotting problem on wood free paper generally associated with conventional ink compositions.